This invention generally relates to a control strategy for avoiding tooth butt conditions in a vehicle transmission. More particularly, this invention relates to a clutch control strategy for overcoming tooth butt situations.
In vehicle drive lines, there typically is an engine that provides a driving force to wheel axles through a transmission that allows for selective gear ratios to be established between the engine output shaft and the wheel axles. Many arrangements include a clutch between the engine and the transmission for selectively coupling the output from the engine to the input to the transmission.
One difficulty associated with operating such systems is accommodating proper engagement of the gear members within the transmission gear box. There are a variety of situations where particular control is required to achieve a successful shift between gear ratios.
One situation that may be encountered during a shift is the so-called tooth butt condition. In order for gear members to be properly meshed, the teeth on one gear member must fit within the spacings between teeth on a cooperating gear member. At times, the gear members are aligned in a manner that the teeth abut against the teeth on the cooperating gear members rather than fitting into the spacings between the teeth. This condition must be dealt with or the desired gear cannot be engaged. This is especially likely when a shift is attempted while the vehicle is not moving.
Traditional methods of overcoming a tooth butt situation include repeatedly engaging the clutch and then releasing the clutch in an attempt to provide some rotation to the input shaft of the transmission to move the gear members relative to each other so that the teeth are no longer aligned. There are several shortcomings and drawbacks associated with this approach.
One difficulty is that the optimum target position for how far the clutch should be closed varies between different transmissions, different vehicles and different operating conditions. Factors such as oil temperature, tolerances and torque curves all have an effect on the correct or optimum target clutch position. Conventional arrangements do not accommodate such variations.
Because of the varying amount of clutch closure needed between transmissions or vehicles, the conventionally calibrated target position of a clutch to avoid a tooth butt condition is set undesirably high for some situations. While a consistent target clutch position may allow for proper meshing and full engagement under most situations, it can result in degraded shift comfort or unwanted transmission faults. For example, the vehicle driver may experience a bump upon engagement. Conversely, setting the consistent target clutch position at a low value may prevent proper engagement under many conditions for some transmission models.
There is a need for an improved strategy for overcoming a tooth butt condition without adding complexity or cost to the system. This invention addresses that need while avoiding the shortcomings and drawbacks of the prior art.
In general terms, this invention is a strategy for overcoming a tooth butt condition by varying a target clutch position over time so that successful gear engagement becomes possible.
In one system designed according to this invention, a clutch is associated with a transmission that includes a plurality of selectively engageable gear members. The clutch selectively couples an engine with the transmission. A controller determines when a tooth butt condition exists and automatically closes the clutch to a target position to provide some engagement between the engine and the transmission. The controller varies the target clutch position over time until a successful gear shift is completed or the controller determines that a selected criteria is met.
One method of overcoming a tooth butt condition designed according to this invention includes determining that a tooth butt condition exists. A clutch is then closed to a target position in an attempt to move at least one of the gear members relative to the other by selectively coupling the engine to the transmission for a limited time. If the tooth butt condition is not yet eliminated, then the clutch is closed to a second, different target position to provide a different level of coupling between the engine and the transmission. This process preferably is repeated a sufficient number of times to overcome the tooth butt condition or until a selected criteria is satisfied.
A variety of factors may be taken into account for modifying or customizing the manner in which the different target clutch engagement positions are set.
The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiments. The drawings that accompany the detailed description can be briefly described as follows.